Enhancing the Hydrogen Embrittlement Resistance of Medium Mn Steels by Designing Metastable Austenite with a Compositional Core–shell Structure

Abstract

Deformation-induced martensite transformation from metastable retained austenite is one of the most efficient strain-hardening mechanisms contributing to the enhancement of strength-ductility synergy in advanced high-strength steels. However, the hard transformation product (often $\alpha^{\prime}$-martensite) and the H redistribution associated with phase transformation essentially decrease materials’ resistance to hydrogen embrittlement. To solve this fundamental conflict, we introduce a new microstructure architecting strategy based on an accurately design of core–shell compositional distribution inside the austenite phase. We employed this approach in a typical medium Mn steel (8 wt.% Mn) with an ultrafine grained austenite-ferrite microstructure. We produced a high Mn content (15–16 wt.%) in the austenite shell region and a low Mn content (~ 12 wt.%) in the core region, through a thermodynamics-guided two-step austenite reversion treatment. During room-temperature deformation, the austenite core transforms continuously starting from a low strain, providing a high and persistent strain-hardening rate. The transformation of Mn-rich austenite shell, on the other hand, occurs only at the latest regime of the deformation, thus effectively inhibiting the nucleation of H-induced cracks at ferrite/deformation-induced martensite interfaces as well as suppressing their growth and percolation. This step-wise transformation, tailored directly targeted to protect the hydrogen-sensitive microstructure defects (interfaces), results in a significantly enhanced hydrogen embrittlement resistance without sacrificing the mechanical performance in hydrogen-free condition. The design of compositional core–shell structure is expected to be applicable to, at least, other multiphase advanced high-strength steels containing metastable austenite.